The present disclosure relates to control rods for controlling a nuclear reactor.
A pressurized water reactor (PWR) generally employs a central core containing fissile nuclear fuel assemblies or bundles of nuclear fuel rods that contain fissile material. Thermal energy generated by the fissile reaction heats primary coolant, which is typically light water (H2O) optionally including additives such as boric acid or another soluble neutron poison, although other coolants/moderators such as heavy water (D2O) are also contemplated. The primary coolant passes through a steam generator where it transfers heat to a secondary coolant (usually water), turning the water into steam. The steam can subsequently be used to operate a turbine to generate electrical power or can be used for another purpose. Other types of nuclear reactors operate similarly. For example, in boiling water reactors (BWR) the primary coolant/moderator is not as highly pressurized but is allowed to boil and produce steam directly.
Control rods are inserted into or removed from the core to control the neutron population density of the fuel assemblies. The control rods are fastened at their top ends to a spider assembly. The control rod typically comprises a stainless steel cladding surrounding a neutron-absorbing material, such as an alloy of silver-indium-cadmium (Ag—In—Cd), boron carbide (B4C), or hafnium (Hf) metal. The control rods are slid into and out of guide tubes that are located within the fuel assemblies.
When using hafnium, one consideration that must be taken into account is hydriding. Hydrogen, for example from the reactor coolant, may diffuse through the stainless steel cladding and react with hafnium to form hafnium hydride (HfH2). This is a concern because HfH2 has a greater volume than that of the Hf metal in the original control rod. Swelling of the control rod thus occurs when the Hf metal is converted to HfH2. This may cause problems, depending on the location of and extent of the swelling, that affect the safety of the nuclear reactor. For example, swelling can increase the amount of time needed to fully insert the control rod into the corresponding the guide tube during a rod scram.
Stainless steel itself is not a strong neutron absorber. The volume occupied by the stainless steel thus decreases the potential reactivity worth of the control rod. The rod worth refers to the neutron-absorbing ability of the control rod. A higher rod worth is desirable. In addition, passive safety concerns dictate that the control rod should be as heavy as is reasonably achievable, so that gravity can be used to insert the control rod into its corresponding guide tube when needed. Stainless steel has a density of around 7.8 g/cc, while hafnium itself has a density of 13.3 g/cc.
It is desirable to provide control rods that have a combination of higher rod worth, increased weight, and greater physical and chemical stability (e.g. no hydride formation as in stainless steel clad Hf rods, or no tritium (H3 or 3H) that is generated in B4C containing rods).